1. Field of the Invention
This invention relates to a method of and device for restraining the rotational motion of articles, for example, glass sheets, automotive backlites or automotive windshields, during transit by applying forces of different magnitude to selected portions of the articles.
2. Discussion of the Prior Art
Glass articles, e.g. glass sheets, automotive backlites or automotive windshields, are normally shipped to automotive manufacturers in bins or racks. To prevent damage to the glass articles during shipping, several expediencies are employed.
The windshields are normally loaded in racks in a generally vertical position with an edge of the windshields resting on a resilient pad and the windshields tilted toward and resting on back support members. Corrugated cardboard spacers are inserted between each windshield to prevent surface contact between adjacent windshields which could mar the windshield surface. A restraining system such as polyester webbing or steel bands is used to urge the windshields toward the back support members. The racks are loaded either on freight cars or trucks for shipment to automotive manufacturers.
During transit, the swaying motion of the freight car imparts a moment of force to the windshields which rotates the windshields about the edge resting on the pad. The windshields oscillate about the edge which causes the webbing to stretch, thereby increasing the path of oscillation. As the path of oscillation increases, the force of the windshields hitting the back support members increases which causes the windshields to break.
U.S. Pat. No. 2,953,253 discloses a windshield container having a restraining system that eliminates the drawbacks associated with polyester webbing or steel bands, but has limited use. In general, the container is provided with a plurality of spaced, stationary, upright members and a plurality of spaced, movable, upright members. Each of the stationary upright members is spaced from one of the movable upright members to form slots for receiving individual windshields. The movable upright members, e.g. a pair of spaced pressure pads, are urged toward their adjacent stationary members about the windshields to prevent motion of the windshields during transit.
The restraining system of the above-mentioned patent is acceptable for restraining individual windshields but not for restraining a plurality of windshields because the force applied by the pair of spaced pressure pads is uniform. As can be appreciated, the moment of force required to rotate a generally flat object about an axis decreases as the distance from the axis of rotation increases. Applying uniform forces to different portions, i.e., a top and bottom section of the windshields, to prevent rotation thereof puts a bending moment on the windshields which can cause them to crack. More particularly, if the forces applied to the windshields are equal and of sufficient magnitude to overcome the resultant moment of force acting on the section of the windshield closest to the axis of rotation, insufficient force will be applied to the section of the windshield farthermost from the axis of rotation. If the forces applied are equal and of sufficient magnitude to overcome the resultant moment of force acting on the portion farthermost from the axis of rotation, excessive force will be applied to the section of the windshield closest to the axis of rotation. "Resultant moment of force", as the term is used herein, is the difference between the moment of force acting at points on the articles which tend to rotate the articles in one direction less the force generated by the articles to resist the rotation, e.g. the angle of tilt and the weight of the articles.
In each instance a bending moment is imparted to the windshields that could cause the windshields to crack. As the number of articles increases, the applied force needed to prevent rotation increases, thereby increasing the bending moment. For this reason, a restraining system that applies equal forces at various distances from the axis of rotation such as the one disclosed in the above-mentioned application is not acceptable for restraining the rotational movement of a plurality of windshields.
U.S. Pat. No. 2,156,876, assigned to PPG Industries, Inc., discloses a safety rack in which glass plates are loaded in a vertical position on one edge and rest on a back support. The glass plates are separated by wire spacers. The plates are urged against the back support by an upright arm carrying a block adapted to engage the face of the outermost plate on the rack. The arm is pivotally mounted on a shoe which freely rides on a bar toward and away from the glass sheets. The arm and block are maintained in position by the corner of the arm engaging the bar.
This type of container has limitations for shipping glass plates because the vibration encountered during transit tends to disengage the corner of the arm from the bar moving the block away from the glass sheets. When this occurs, the path of oscillation of the glass increases which can cause the glass to be damaged.